What does “black” mean in this case?
Since it's written by Doyle, at the time he was living, you can be pretty sure she was not a negro.
Coloured people were quite unusual in the streets of England at the time - compared to nowadays. If she had been what we would call black now, he would have said so, and maybe with words no longer in polite usage.
In that century someone with jet black hair, dark eyes and maybe sallow skin too, would be described as black. Think also of Black Irish.